The present invention relates to rotor position estimation, and more particularly, to optical sensor and method for estimating positions of rotors in a motor and to the motor comprising the optical sensor.
To drive a permanent magnet synchronous motor (PMSM), the rotor position is needed to allow field oriented control (FOC). A rotor position sensor such as resolver, hall sensor, GMR sensor, or optical encoder can provide the rotor position information, but it is usually costly.
There have been a lot of researches on sensorless control topic which have intended to use the 3-phase current information to extract the rotor position information and therefore to relieve the system from rotor position or angular position sensors so that total cost is reduced and system reliability is improved.
Specifically, in a situation without rotor position sensor, the rotor position can be estimated by extracting the position information from the measured phase currents. Typical extraction methods include non-linear observer method, or sliding-mode observer method.
However, most of the methods involve low-pass filter and there are delay times involved. When the rotational speed is very high (typically more than 6000 rpm), the delay time will be comparable to the electrical rotational cycle. The deficiencies of the conventional techniques may be clearly understood from FIG. 1, which shows the delay time between actual speed ωm and expected speed from the test result when sensorless control is applied. In this case the sensorless control algorithm has limit to extract the rotor position information in the rotational cycle when the speed is up to a level of more than 6000 rpm typically for a pole pair of 4 and the limit level would be lower if pole pair is bigger.
Prior art researches or inventions have addressed the problems when the e-machine is at standstill or at low speed when the sensorless control strategy has few limits or accuracy issues, but no prior arts have taken high speed issues into consideration.